Solve for $z$ : $7 = 4 + z$
Subtract $4$ from both sides: $ \begin{eqnarray} \\ \hphantom{7 {- 4}} &\hphantom{=}& \hphantom{z} \\ 7 &=& 4 + z \\ \\ {-4} && {-4} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 7 {- 4} &=& z \\ \end{eqnarray} $ Simplify. $ z = 3$